1Integrative Biology, University of California, Berkeley, 2Anthropology, University of Texas, Austin, 3Anthropology, University of Michigan, Ann Arbor
April 18, 2020 4:45PM, Diamond 3
Human attraction to ethanol has been hypothesized to be evolutionarily rooted through prolonged ancestral exposure to low dietary concentrations within fermenting fruit. Fermentative yeasts are ubiquitous, vectored by insects and dust motes, and may be present within saccharide-rich substrates without outward signs of decomposition. Obligately frugivorous taxa such as chimpanzees consume large quantities of fruit in relation to their body size, amounting to several kilograms daily. Thus, even low ethanol concentrations potentially add up to a pharmacologically significant dosage, equivalent to multiple standard drinks in human terms. Although ethanol concentrations have been previously documented for primate-consumed fruit, ethanol concentrations within the diet of chimpanzees, our species’ nearest relative, have not been systematically examined. Here we look at Ficus mucuso, the food item most consumed annually by eastern chimpanzees at Ngogo, Kibale National Park, Uganda. Like most Ficus species, F. mucuso is pollinated by specialized fig wasps that lay their eggs within the inverted infructescence. Ethanol concentrations were estimated for over 200 figs using field portable near-infrared spectrometry and gas chromatography. Unripe figs (i.e. those with unhatched wasps) had higher ethanol concentrations (1.1% n=35) than those free of wasps (0.5%, n=150) and hanging figs (preferred by chimpanzees) had higher concentrations (0.8% n=85) than fallen ones (0.4%, n=124). Assays of small sample sizes of several other species of chimpanzee-consumed fruits test positive for presence of ethanol. These results have broader ecological implications for other primate and non-primate taxa and are consistent with prolonged evolutionary exposure to dietary ethanol amongst all hominoid taxa.